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Catch it if you can – Quantifying Runoff

Soil Texture Triangle: After identifying the percent , and in your sample, use the texture triangle below to identify your . To do this, follow the straight or diagonal line for each soil separate, using the angle of the separate to identify which line to follow (ex: for silt and sand you will be following a diagonal line). Your classification is determined by the intersection of the 3 lines in the triangle. For example, if your sample has 50% clay, 30% silt, and 20% sand, then your sample is "clay."

(This figure is in the public domain from the USDA Natural Conservation Service) http://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/nedc/training/soil/?cid=nrcs142p2_054167

 Clay is a very fine . When it is dry it is very hard, and what it is wet, water cannot flow through.  Sand provides soil with air spaces and pathways for water to flow  Silt is a light particle that erodes easily, but makes for good farm .  is a "perfect soil" that is an even mix of all three soil separates.

Description of Hydrologic Soil Groups

HSG Soil Textures A Sand, loamy sand, or sandy loam B Sandy clay loam C Silt loam or loam D Clay loam, silty clay loam, sandy clay, silty clay, or clay Table of Curve Numbers (Adapted from USDA Natural Resources Conservation Service)

Cover description Hydrologic Soil Group Cover type and hydrologic condition A B C D —grass combination ( or tree farm) 43 65 76 82 Woods 36 60 73 79 Fallow - Bare soil 77 77 86 94 Residential districts by average lot size: 1/8 acre or less (town houses) 77 85 90 92 1/4 acre 61 75 83 87 1/3 acre 57 72 81 86 1/2 acre 54 70 80 85 1 acre 51 68 79 84 2 acres 46 65 77 82

The curve numbers refer to the permeability of the soil types based upon type. A curve number of "100" is interpreted as a surface that is 100% impervious. Included in this table are only a small sample of the curve numbers available to calculate runoff; however we have what we need to analyze the problem in this lesson. Look at the numbers above and identify any trends in the data and account for those trends as related to soil type and surface cover type.

If you would like to apply the content of this problem to your own problem, you can find the additional information you need at the following links:

Web – to identify the Hydrologic Soil Group if access to site is not possible http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

All the Curve Numbers may be found in "Urban for Small Watersheds TR-55" from the USDA- NRCS and published in 1986. A web search will bring up the PDF document.